Today, as a consequence of greater social and health awareness with consumers, there is a high demand in the wine world for solutions to decrease the alcohol content in the final wine. As an alternative to full-strength wines, wines with a reduced alcohol content offer a number of potential social and health benefits for consumers. Social benefits may include improved productivity and function after activities involving alcohol (e.g. business lunches), lower risk of accidents while driving and more acceptable social behavior in general. Health advantages may include reduced calorie intake, decreased risk of alcohol-related illness and disease and specific benefits for pregnant women. For the producers of these wines, there exists an incentive of identified markets and market segments, as well as lower sales and duty taxes applicable in many countries (Pickering, 2000).
Methods for producing reduced-alcohol wines have been available since the early 1900s. Although commercial production nowadays relies almost exclusively on systems based on the use of membranes and modified distillation, a number of alternative approaches have been put forward in literature. These approaches are summarized in Table 1 (Pickering, 2000).
TABLE 1Current techniques for producing reduced-alcohol wines (adaptedfrom Pickering, 2000 and Schmidtke et al. 2012)PrincipleMethodReduction ofUse of unripe fruitfermentable sugarJuice dilutionconcentration inFreeze concentration and fractionationgrape or juiceEnzymes (e-g- glucose oxidase)Removal of alcoholThermal: distillation, evaporation or freezefrom wineconcentrationMembrane: dialysis or reverse osmosisAdsorption: resins or silica gelExtraction: organic solvents or supercritical CO2Modified yeast strainsOtherDilution of wineArresting fermentation earlyLow-alcohol-producing yeastCombinations of above methods
When using unripe fruit and/or diluted juice/must, the flavor of the final wine is compromised, due to the lower concentration of flavor precursors. Mechanical removal of ethanol tends to be a harsh process that affects the overall chemical composition of the wine, and thereby also the sensory profile and experience of the wine.
A lot of work has been done so far in reducing sugar with a biological approach. Five different methods have been explored: 1) aeration of the must during fermentation using Saccharomyces sp. with a conversion of sugar to CO2, water and biomass, 2) aeration of the must during fermentation using non-Saccharomyces sp. with a conversion of sugar to CO2, water and biomass, 3) the use of non-Saccharomyces and Saccharomyces sp. in a sequential fermentation, 4) early arrest of fermentation for the production of sweet wines and 5) genetically modified Saccharomyces cerevisiae wine strains to redirect ethanol production.
Lowering alcohol percentage by aeration of the must during fermentation using Saccharomyces sp. has been explored since 1989. A process has been described for the preparation of low-sugar or sugar-free fruit juices based on continuous or semi-continuous culture with yeast, with the conditions resulting in metabolism of sugar to CO2 and water rather than to ethanol and so recovering an alcohol-free, sugar-free or low sugar juice by separation of the biomass (Kaeppeli, 1989). A similar system is described by Grossmann et al. (1991) for producing low-alcohol drinks, where oxygen is added to the fermenting liquid in a controlled manner in order to convert the sugars to water and CO2. The supply of oxygen is interrupted when the required level of residual alcohol is reached, then the yeast is separated from the liquid, and finally the liquid is microfiltered in order to produce a clear liquid product.
By screening a broad collection of yeast species, Kolb et al (1993) found that Pichia stipitis was particularly well suited for juice sugar removal. Their claims include the elimination of more than 50% of juice sugar within 20 h, and a minimum of adverse effects on the sensory and functional qualities of the juice. Similar results have been obtained by investigating the effect of varying aeration and temperature levels on the reduction of sugar and production of alcohol in Muller-Thurgau grape juice by selected yeast strains (Smith et al. 1995). Seven yeast strains showed promising results of which three strains produced significant alcohol reductions: Pichia stipitis, Candida tropicalis and Saccharomyces cerevisiae. By combining short-term controlled aeration, to reduce the sugar content, with anaerobic fermentation using an active dried wine yeast, wines with acceptable taste and 25 to 30% less alcohol were produced. However, these wines exhibited the deep golden color indicative of oxidation, which generally is undesired in wine.
More investigations into mixed starter cultures of non-Saccharomyces species with a Saccharomyces cerevisiae wine strain revealed that the alcohol percent (v/v) could be reduced with 0.2-0.7% (Ciani and Ferraro, 1996; Ferraro et al. 2000; Erten and Campbell, 2001; Ciani et al. 2006; Garcia et al. 2010; Maygar and Toth, 2011; Comitini et al. 2011; Di Maio et al. 2012; Sadoudi et al. 2012). In a recent publication of the Australian Wine Research Institute (AWRI), evaluation of 50 different non-Saccharomyces isolates, belonging to 24 different genera, has been performed for their capacity to produce wine with lower ethanol concentration when used in sequential inoculation regimes with a Saccharomyces cerevisiae wine strain (Contreras et al. 2013). A sequential inoculation of Metschnikowia puicherrima AWRI1149 followed by a Saccharomyces cerevisiae wine strain was the best combination able to produce wine with a lower ethanol concentration than the single-inoculum, wine yeast, control. Sequential fermentations utilizing AWRI1149 produced wines with 0.9% (v/v) and 1.6% (v/v) (corresponding to 7.1 g/L and 12.6 g/L) less ethanol concentration in Chardonnay and Shiraz, respectively.
Another approach of lowering ethanol in wine production is the use of genetically modified yeast strains (Kutyna et al., 2010). However, with the current consumer reluctance against the use of GMO yeast strains in wine, this option is not considered to be commercially viable at this moment.
Currently, the mentioned methods have a limited commercial application. The main problem is the reduced sensory quality compared with full-strength wine. In particular, problems with flavor imbalance and a lack of ‘body’ have been reported. The main flavor compound resulting in an imbalanced flavor profile is ethyl acetate, which is often found in very high concentrations in the reduced alcohol wines produced with the current methods. These altered sensory properties can occur as a result of the processing required to produce the reduced alcoholic wine or as a direct consequence of the reduced ethanol content. The main characteristics of the biological approaches to reduced alcohol wines is the potential off-flavor development (too high acetic acid or ethyl acetate concentrations) with the use of non-Saccharomyces yeast strains or oxidation of the final wine when aeration techniques are used, resulting in undesirable organoleptic characters.